Global challenges Revision Notes

Alkanes: Saturated Hydrocarbons

Alkanes are hydrocarbons containing only carbon-carbon single bonds. They are saturated compounds with the general formula CnH2n+2. The first four members are:

1. Methane (CH4): One carbon atom.
2. Ethane (C2H6): Two carbon atoms.
3. Propane (C3H8): Three carbon atoms.
4. Butane (C4H10): Four carbon atoms.

When drawing fully displayed formulae, show all C-H and C-C single bonds. For example, ethane would be H-C(H)(H)-C(H)(H)-H, showing how each carbon is bonded to three hydrogens and one other carbon.

Alkenes: Unsaturated Hydrocarbons

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. They are unsaturated compounds with the general formula CnH2n (for molecules with one double bond). The first four members typically considered are:

1. Ethene (C2H4): Two carbon atoms, with a C=C double bond.
2. Propene (C3H6): Three carbon atoms, with one C=C double bond.
3. But-1-ene (C4H8): Four carbon atoms, with a C=C double bond between the first and second carbon.

When drawing, clearly show the C=C double bond and all C-H single bonds. Remember that each carbon atom must have a total of four bonds.

Alcohols: Containing the Hydroxyl Group

Alcohols are organic compounds that contain the hydroxyl functional group (-OH) attached to a carbon atom. Their general formula is CnH2n+1OH. The first four members are:

1. Methanol (CH3OH): One carbon atom.
2. Ethanol (C2H5OH): Two carbon atoms.
3. Propan-1-ol (C3H7OH): Three carbon atoms. The -OH group is on the first carbon.
4. Butan-1-ol (C4H9OH): Four carbon atoms. The -OH group is on the first carbon.

When drawing fully displayed formulae, show all C-C, C-H, C-O, and O-H bonds clearly. The oxygen atom in the hydroxyl group forms two bonds.

Carboxylic Acids: Containing the Carboxyl Group

Carboxylic acids are organic compounds characterized by the carboxyl functional group (-COOH). This group consists of a carbonyl (C=O) and a hydroxyl (-OH) group bonded to the same carbon atom. Their general formula is CnH2n+1COOH. The first four members are:

1. Methanoic acid (HCOOH): One carbon atom (the carbon of the carboxyl group).
2. Ethanoic acid (CH3COOH): Two carbon atoms.
3. Propanoic acid (CH3CH2COOH): Three carbon atoms.
4. Butanoic acid (CH3CH2CH2COOH): Four carbon atoms.

When drawing, ensure the double bond for C=O and the single bonds for C-O and O-H within the -COOH group are clearly shown, along with all other bonds.

What are Alloys?

An alloy is a mixture composed of two or more elements, where at least one of these elements is a metal. The elements are typically melted together and mixed to form a uniform structure upon solidification. Pure metals often consist of layers of identical atoms that can slide over each other easily, making them relatively soft and malleable. The introduction of different sized atoms (from the added element) into the metallic structure of an alloy disrupts this regular arrangement. This disruption makes it more difficult for the layers of atoms to slide past one another, resulting in alloys generally being harder and stronger than the pure metals from which they are made.

Properties and Uses of Steel

Steel is a widely used alloy primarily composed of iron and carbon. The small carbon atoms fit into the spaces between the larger iron atoms, hindering the movement of iron layers. This makes steel significantly harder and stronger than pure iron. Various types of steel exist, each with specific properties tailored for different uses. For example, mild steel (low carbon content) is strong and tough, used for car bodies and structural girders. High carbon steel is very hard but brittle, suitable for cutting tools like chisels. Stainless steel, an alloy of iron with chromium and nickel, is known for its excellent corrosion resistance due to a protective oxide layer that forms on its surface. This property makes it ideal for surgical instruments, cutlery, and kitchen sinks.

Properties and Uses of Other Important Alloys

Brass is an alloy of copper and zinc. It is harder and stronger than pure copper, and it has an attractive appearance and good workability. Brass is widely used for musical instruments, decorative items, and plumbing fittings. Bronze is an alloy of copper and tin; it is even harder and more corrosion-resistant than brass, making it suitable for statues, bells, and strong, durable components like ship propellers. Duralumin is an important alloy of aluminium, typically containing copper, magnesium, and manganese. While aluminium is lightweight, duralumin retains this property but is much stronger, making it invaluable in aerospace industries for aircraft construction where both lightness and high strength are crucial.

Why Alloys Have Different Properties

The altered properties of alloys compared to their pure metallic components are directly linked to their atomic structure. In a pure metal, atoms are typically of similar size and arranged in regular, repeating layers. When stress is applied, these layers can slide past each other relatively easily, leading to the metal being ductile and malleable. In an alloy, the presence of different-sized atoms from the added element distorts this regular arrangement. These foreign atoms act as 'obstacles', pinning the layers in place and preventing them from sliding easily. This 'dislocation pinning' mechanism is responsible for the increased hardness and strength observed in most alloys. By carefully controlling the type and proportion of added elements, specific properties can be engineered, such as enhancing strength, improving corrosion resistance, or altering melting points for particular applications.

Understanding Rusting and Corrosion

Corrosion is the general term for the destructive attack of a metal by reaction with substances in its environment, such as oxygen and water. For iron and steel, this specific corrosion process is known as rusting. Rusting requires both oxygen and water to be present simultaneously; if either is absent, iron will not rust. Rust is hydrated iron(III) oxide. Preventing corrosion usually involves stopping one or both of these reactants from reaching the metal surface or using a more reactive metal to divert the corrosion process.

Physical Barrier Protection

One way to mitigate corrosion is by creating a physical barrier. This involves coating the metal with a protective layer that physically prevents oxygen and water from coming into contact with the metal surface. Common barrier methods include painting, oiling, greasing, or coating with plastic. For example, painting a metal garden gate creates a waterproof and air-tight layer. While effective, a key limitation of physical barriers is that if the coating is damaged (e.g., scratched, chipped), the underlying metal becomes exposed and will then begin to corrode at that point, as oxygen and water can access it.

Sacrificial Protection (Galvanising and Sacrificial Anodes)

Sacrificial protection involves connecting a more reactive metal to the metal you want to protect. The more reactive metal will corrode preferentially (sacrifice itself), thereby protecting the less reactive metal. A common example is galvanising, where iron or steel is coated with a layer of zinc. Zinc is more reactive than iron. If the galvanised layer is scratched, exposing both metals, the zinc will oxidise in preference to the iron, protecting the iron from rusting. This protection continues as long as there is zinc present. Other reactive metals like magnesium or aluminium can be attached as sacrificial anodes to larger iron or steel structures (e.g., pipelines, ship hulls) which slowly corrode away, protecting the main structure.

Carbon Monoxide (CO)

Carbon monoxide is a colourless, odourless, tasteless, and highly toxic gas produced by the incomplete combustion of carbon-containing fuels (e.g., petrol, diesel, coal, gas). Major sources include vehicle exhausts (especially in heavy traffic or poorly maintained engines) and faulty or poorly ventilated heating appliances within homes. The danger lies in its ability to bind irreversibly with haemoglobin in red blood cells, preventing oxygen transport around the body. This leads to oxygen deprivation in tissues and organs, causing symptoms like headaches, dizziness, nausea, and in severe cases, unconsciousness and death.

Sulfur Dioxide (SO2)

Sulfur dioxide is primarily released during the combustion of fossil fuels, particularly coal and some types of crude oil, which contain sulfur impurities. Power stations burning these fuels are significant sources. Once released into the atmosphere, SO2 can react with water vapour and oxygen to form sulfuric acid (H2SO4), which then falls as acid rain. Acid rain causes extensive damage to buildings and statues (especially those made of limestone or marble), acidifies lakes and rivers, harming aquatic life, and damages forests and other vegetation by making soil more acidic and leaching vital nutrients.

Oxides of Nitrogen (NOx)

Oxides of nitrogen, mainly nitrogen monoxide (NO) and nitrogen dioxide (NO2), are formed when nitrogen and oxygen from the air react at the high temperatures found in internal combustion engines (e.g., cars, lorries) and power stations. NO2 is a reddish-brown gas with a pungent smell. Like sulfur dioxide, NO2 contributes to acid rain by forming nitric acid (HNO3) when it reacts with atmospheric water. NOx gases also play a key role in the formation of photochemical smog, which can irritate the respiratory system and eyes, and exacerbate respiratory diseases like asthma.

Particulates

Particulates are tiny solid particles or liquid droplets suspended in the atmosphere. They are produced from various sources, including the incomplete combustion of fossil fuels (e.g., diesel engines, wood stoves), industrial processes, and natural sources like dust and sea spray. Examples include soot (carbon particles). Particulates pose health risks, particularly to the respiratory and cardiovascular systems. Inhaling fine particulates can lead to or worsen conditions such as asthma, bronchitis, and heart disease. They also reduce visibility, contributing to haze, and can deposit on surfaces, causing soiling and damage.